Empty unit tests updating to Swift 2.3 / Xcode 8

I'm currently upgrading to Swift 2.3 with Xcode 8. Despite the fact that my unit tests have been left totally empty before and after upgrading, I am now getting errors there. The error is "Module file was created by an older version of the compiler; rebuild 'Project' and try again". What is the issue here? I have installed the Swift 2.3 version of my CocoaPods pods and run the migrator program on my own code.